The invention relates to improvements in methods of and in apparatus for treating running webs with fluid media, especially for treating running webs of moist fibrous material with steam.
Steam distribution methods and apparatus which are used to promote the rate of drainage of water from sheets or webs of fibrous material in paper making machines are disclosed, for example, in U.S. Pats. Nos. 4,351,700 and 4,444,622 to Dove. Condensate which deposits on the running web as a result of contact between the web and steam raises the temperature of the web which, in turn, promotes drainage of water from the web ahead of the drying zone where the remaining water must be expelled by evaporation. Moreover, the rate at which longitudinally extending portions of the web are contacted with steam can be regulated to impart to the web a preselected moisture profile ahead of the drying station.
The housing of the patented steam distribution apparatus can be disposed above or below the path of the running web or adjacent a convex side of such path, depending upon the selected location of the apparatus in a paper making machine. Thus, if the distribution apparatus is placed next to the nip of a Yankee cylinder with a suction press roll, its steam discharging wall has a concave side which faces the adjacent convex side of the running web of moist fibrous material. The housing of the distribution apparatus will be placed above the path of the web in a straight through press or in a Fourdrinier machine and beneath the path of the web in the suction transfer zone of a paper making machine. Regardless of the location of the apparatus, the latter is designed to develop a so-called steam curtain at its upstream end, and the purpose of the steam curtain is to prevent penetration of surrounding atmospheric air into the treating zone between the orifices of the housing of the distribution apparatus and the adjacent side of the running web. The steam curtain at the upstream side of the apparatus is formed by discharging steam at a higher pressure than the pressure of steam which is discharged between the upstream and downstream sides. The establishment of a steam curtain is deemed necessary because air is a non-condensable gas and, according to the patentee, the presence of air in the zone where the bulk of steam impinges upon the running web would reduce the rate of steam condensation by a factor of the order of 4 to 1. The outer side of the apertured wall at the trailing edge of the apparatus which is disclosed in U.S. Pat. No. 4,351,700 has a convex shape and the jets of steam which issue therefrom are directed forwardly, i.e., in the direction of advancement of the running web. In each of the aforementioned patents, the inventor emphasizes the need to prevent penetration of air into the region between the housing of the apparatus and the path for the running web.